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Abstract

This work presents an apparatus that measures near-forward two-dimensional elastic scattering patterns of single aerosol particles and proposes a two-angle extension of the Guinier law to analyze these patterns. The particles, which approximately range from 2 to 8 micrometers in size, flow through the apparatus in an aerosol stream. A spatial filtering technique separates the near-forward portion of the patterns from the illumination light. Contours intended to represent the geometrical profile of the particles are generated from the patterns using the extension of the Guinier law. The analysis is applied to spherical and nonspherical particles, and the resulting contours are found to be consistent with particle shape only for spherical particles.

Figures (5)

Schematic of the apparatus used to collect single aerosol-particle scattering patterns. Particles are generated from aqueous solutions by a ROYCO aspirator or from dry powder by an Erlenmeyer generator, see top insets. The particle flow is focused by a nozzle to the apparatus’ scattering volume. There a crossed beam trigger system senses a particle and activates a pulsed Nd:YAG laser. The 2D near-forward scattering pattern is separated from the illuminating Nd:YAG light (second-harmonic, 532 nm) by the spatial filter and imaged onto a CCD camera. A block diagram is shown that describes the electronic signal processing involved in the trigger system, see text for further description.

Scanning electron microscope images of the aerosol particles. Image (a) shows a sodium chloride crystal, (b) crystallized riboflavin particles, and (c) a 4.3μm polystyrene latex microsphere. The salt crystal and microsphere are captured directly from the aerosol stream by placing the SEM specimen holder below the aerosol nozzle. The riboflavin image, however, is a dried droplet of solution on the SEM holder; difficulties associated with their capture prevent SEM imaging of the particles while in the aerosol stream.

Two-dimensional scattering patterns collected from single aerosol particles using the apparatus shown in Fig. 1. The particle-illuminating laser light travels through page toward the reader and is polarized vertically. A description of the particles producing these patterns is given in the text.

Contour generation resulting from the two-dimensional extension of the Guinier law. The scattering pattern on the left is pattern (g) in Fig. 3 represented here in the qx–qy plane. The contour on this pattern represents the locus of points determined by the scattering angle θ at which the intensity decreases to 2/3 of its value in the forward direction. The plot on the right shows the contour resulting from the inverse of these points as described in the text.